Screw rotor machines employable, both for the compression or expansion of an elastic working fluid, have used asymmetric rotor profiles for improved efficiency of the compression or expansion process. Exemplary U.S. Patents include U.S. Pat. Nos. 3,423,017, 4,140,445 and 4,435,139, assigned to Svenska Rotor Maskiner: U.S. Pat. No. 4,053,263, U.S. Pat. No. 4,109,362 and 4,445,831 issued to the applicant and assigned to Joy Manufacturing Company; U.S. Pat. No. 4,527,967 issued to the applicant and assigned to Dunham-Bush. Incorporated: U.S. Pat. Nos. 4,401,420 and 4,406,602 assigned to Hitachi Corporation; U.S. Pat. No. 4,412,796 and U.S. Pat. No. 4,508,496 assigned to Ingersoll-Rand Company: and U.S. Pat. No. 4,028,026 assigned to Linde Aktiengesellschaft.
Screw rotor machines, whether functioning as compressors or expanders, are normally of a cast or machined casing or housing bearing two parallel, laterally intersecting cylindrical bores opening at respective ends to high and low pressure ports. Within the bores there are mounted for rotation, interengaging helical screw rotors of the male and female type respectively, provided with helical lobes or lands and intervening grooves having wrap angles normally less than 300.degree.. Typically the male rotor is a rotor in which each lobe and groove has at least its major portion located outside the pitch circle of the rotor and has two generally convex flanks located outside of the pitch circle. The female comprises a rotor in which each lobe and groove has at least its major portion located inside of the pitch circle and has two generally concave flanks located inside the pitch circle of the rotor.
The known patents covering asymmetric screw rotor machine utilize male and female rotor profiles which profiles are described by use of radial lines, arcs of circles, ellipses, trochoids, parabolas, involutes, hyperbolas and other geometric shapes forming portions of the lobe surfaces.
The manufacture of asymmetric screw rotor profile is not only complex, resulting in the use of expensive machinery, but require precise measuring of the surfaces having profiles of generated curves. The difficulty in measuring such surfaces accurately requires heavy dependence on the accuracy of the cutter, grinding and correctness of a Holroyd thread milling machine set up. The result is often the necessity of mating a female rotor with a particular male rotor, maintaining such mating through stocking and assembling procedures and leading to the necessity for replacing both rotors should one rotor be damaged. Attempts have been made in the past to utilize involute curves in the profiling of both the male and female screw rotors. Involute curves on the trailing lobe flanks of the male rotors and the trailing groove flanks of female rotors as set forth in applicant's U.S. Pat. Nos. 4,053,263, 4,109,362, 4,028,026 and 4,445,831 permit the profiles of those patents to be produced on standard gear cutting or hobbing equipment. U.S. Pat. Nos. 4,412,796 and 4,508,496 employ an involute female leading groove portion and "generated" mating male leading lobe portions.
Additionally, screw rotor machines constructed in accordance with existing current practice are designed to fulfill a wide variety of compressor or expander capacity requirements. Capacity control has been previously accomplished by one or more of several methods. Capacity can be controlled by enlarging or reducing the size of rotors while maintaining basic rotor geometry including the numbers of lobes, male and female profiles and lobe proportions without changing length to diameter ratios (L/D). Alternatively, capacity can be controlled by increasing or decreasing the length to diameter ratios (L/D) of the rotors while maintaining rotor diameter sizes and geometry.
Capacity can also be controlled by increasing or decreasing the rotational speed of rotors without enlarging or reducing the size of the rotors or increasing or decreasing the length to diameter ratios (L/D). This has been accomplished by electrical control, i.e., changing the motor speed and mechanical control by changing the belt and pulley or gear drive from the motor to the compressor rotors.
Where conventionally, the rotors are male rotor driven, the four lobe male rotor, for instance, may drive a six lobe female rotor By using a "female" drive with the female rotor having the larger number of lobes than the male rotor, machine capacity increases by 50%, assuming the same input rotational speeds to the drive rotor. However, it should be appreciated that the female rotor lobes are more fragile than the male rotor lobes and in addition total compressor power is transmitted through the rotor mesh in a "Female Drive" compressor as compared to approximately 16% of total compressor power transmitted through the rotor mesh in a "Male Drive" compressor, and the female rotor lobes may not be able to absorb the stress required as a female drive.
Digital computers have been utilized in the past to compute described geometrical figures for facilitating the creation of a screw rotor profile having sectional flank surfaces of interlinked geometrical figures. They have not to applicant's knowledge been used, however, to develop and define the actual screw rotor profiles for the male and female intermeshed rotors.
It is therefore a primary object of the present invention to provide improved high efficiency screw rotor profile utilizing modern computer technology to determine a series of curves which intersect a grouping of points defined by radial and angular, or by rectangular multiple coordinate dimensions.
It is a further object of the invention to provide a screw rotor machine having improved screw rotor profiles developed by digital computer technology and defined by rectangular or polar coordinates with the actual male and female rotor profile shapes and curvatures determined by "arc fit" programming on digital computers.
It is the further object of this invention to provide helical screw rotor machine utilizing improved screw rotor lobe surfaces common to male and female rotor and employed with different male and female rotor lobe combinations to increase or decrease the capacity of the screw rotor machines.
It is the further object of the invention to provide improved screw rotor profiles using involute curves for the male tip portion and root portion of the male rotor lobe leading flank mating with involute curves on the female rotor groove trailing flank root portion and tip portion, respectively, resulting in mating surfaces therebetween having low leakage paths and particularly applicable to computer program generation characterized by the ability to calculate coordinates for the rotors having a variety of lobe thicknesses and a number of lobes.